Skip CI for PRs on Markdown changes

[huddlej]

Description of proposed changes

Skips the CI workflow when we change the README or change log in a PR. Changes to these files should not produce a different outcome in CI, so we don't need to waste compute rerunning CI for these files. CI will still run for these changes when pushed to master, since the change log gets built as part of our documentation.

Related issue(s)

Checklist

• Automated checks pass
• Check if you need to add a changelog message
• Check if you need to add tests
• Check if you need to update docs

[huddlej]

It looks like this change won't actually be helpful until actions/runner#2324 is fixed. The main use case for this change is to avoid running CI on a PR when we've updated the change log after making other changes to the code. It seems that CI will always run for every commit if any commit exists in the PR that changes a file other than the two ignored here.